1. Field of the Invention
The present invention relates to a toner for use in electrophotographic image forming method adopted in, for example, photocopiers, printers and facsimile machines. The preset invention further relates to a developer comprising the toner, and an image forming method using the toner. The present invention also relates to a method of manufacturing the toner, and an image forming apparatus and a process cartridge using the toner.
2. Discussion of the Background
Recently, the printing and copying volume of graphic documents, such as photographs, has been increasing in addition to that of conventional letter documents with as computers, networking and digitization of information are increasingly used.
Additionally, toner particles have been reduced in size to satisfy demands on improvement in quality of images printed by electrophotography. Further, in order to prepare a toner which can be used for high speed printing, resins having a low softening point are used therefor so that the resultant toner can be quickly respond to heat in fixing. Therefore various kinds of toner manufacturing methods have been studied.
Quality of images printed in electrophotography has been recently improved and is now close to that obtained by using silver-salt films. Therefore, there is an increasing demand for having an average particle diameter of from 4 to 6 μm and a sharp distribution. Commercializing such a toner using a polymerization method has been studied.
Polymerized toners apparently are able to be manufactured with less energy and less burden on the environment, because the amount of CO2 emission is relatively small compared with the conventional toner manufacturing process that includes the steps of kneading, pulverization and classification. However, when a polymerized toner is manufactured, a large amount of water is consumed in the process of granulating particles and a large amount of energy is consumed. Therefore, it is not necessarily the case that a polymerized toner can be manufactured with less burden on the environment. In addition, often a large plant is used for manufacturing a polymerized toner, resulting in increase in its initial cost. Therefore, manufacturing a polymerized toner is not economically feasible unless the same polymerized toner is mass produced for a relatively long period of time.
In addition, with improvement in the function of hardware for use in electrophotography such as copiers, printers and facsimile machines, the toner and developer supplied for such improved hardware have also been improved. However, since a toner manufactured by a polymerization method is difficult to improve except for changing external additives, the toner is not always compatible with the improved hardware. When changing materials used for manufacturing a toner by a polymerization method, manufacturing conditions are usually studied for some time and therefore an immediate adoption of a new toner having a high level of function is difficult.
As for a toner manufactured by a pulverization method, pulverizers capable of efficiently producing a small-sized toner particle having an average particle diameter of from 4 to 6 μm have been studied. It is known to pulverize toner with collision board pulverizers, mechanical pulverizers and counter air pulverizers.
Among the pulverizers mentioned above, mechanical pulverizers and counter air pulverizers are preferably used for manufacturing toners having a small particle.
Mechanical pulverizers are also referred to as impact pulverizers. Particles are pulverized in the mechanical pulverizer when particles collide with each other in a violent current of airflow created by a pulverizing rotor rotating at a high speed.
Specific examples of mechanical pulverizers include KRYPTRON® (manufactured by Kawasaki Heavy Industries, LTD.), TURBO MILL™ (manufactured by Turbo Kogyo CO., Ltd.), and ACM® PULVERIZER and INNOMIZER® (manufactured by Hosokwa Micron Corporation).
Counter air pulverizers are also referred to as air jet pulverizers. Particles are pulverized in counter air pulverizers when particles collide with each other in a counter air jet.
Specific examples of counter air pulverizers include PJM-I (manufactured by Nippon pneumatic MFG. Co., Ltd.), MICRON JET MILL® and COUNTER JET MILL® manufactured by Hosokawa Micron Corporation), and CROSS JET® mill (manufactured by Kurimoto Ltd.).
Among these pulverizers, the counter air pulverizers pulverize particles by collision between particles, and therefore pulverization occurs at particle surface. Thus, the surfaces of particles are shaven and loses their jaggedness, such that substantially round toner particles are obtained. As the contact area of such round toner particles is small, the round toner particles have characteristics such as weak adhesion strength, excellent transferability and good replenishing properties. Considering these points, counter air pulverizes have an advantage over other pulverizers.
However, the inventors of the present invention have studied counter air pulverizers and found that an extremely large amount of super fine powder having a diameter not greater than 2 μm are formed as a result of surface pulverization.
Japanese Patent No. (hereinafter referred to as JP-B) 2896829 discloses a toner capable of producing clear and sharp images, which includes a specific amount of small sized toner particles. It is described therein that the toner is not substantially adhered to a development sleeve, and thereby problems such as background fouling and toner scattering occur.
Specifically, the toner includes toner particles having a particle diameter of from 2.00 to 4.00 μm and from 4.00 to 5.04 μm in an amount of from 3 to 15% by quantity and 8 to 19% by quantity, respectively. JP-B 2896829 also describes that deterioration of image definition can be restrained by appropriately controlling the toner particle distribution.
Further, JP-B 2896826 discloses a toner capable of producing images with high image density and excellent fine line reproducibility and gradation property even when used for image forming apparatus equipped with a toner recycle system. It is described therein that, by using a toner having a specific amount of fine toner particles and coarse toner particles, high quality images can be continuously produced.
Specifically, the toner has toner particles having a particle diameter of from 2.00 to 4.00 μm and from 4.00 to 5.04 μm in an amount of from 3 to 15% by quantity and 8 to 19% by quantity, respectively. Further JP-B 2896826 describes that images with good fine line reproducibility are produced by using a toner including toner particles having a particle diameter not greater than 12.7 μm in an amount of not greater than 10% by volume.
Further, JP-B 2694558 discloses a toner capable of producing images which have high image density and are excellent in fine line reproducibility and highlight gradation.
Specifically, it is described therein that, by using a toner containing toner particles having a particle diameter not greater than 5 μm in an amount of from 8 to 40% by quantity, images with excellent fine dot reproducibility and good image quality can be continuously produced. Further JP-B 2694558 describes that a toner containing toner particles having a particle diameter of from 12.7 to 16.0 μm in an amount of from 0.1 to 15.0% by volume can maintain good fluidity.
Furthermore, JP-B 2763318 discloses a non-magnetic toner for use in a two component developer, which can produce images with high image density, and excellent fine line reproducibility and gradation property.
Specifically, it is described in JP-B 2763318 that the non-magnetic toner includes toner particles having a particle diameter not greater than 5 μm in an amount of from 17 to 60% by quantity, particles having a particle diameter of from 8 to 12.7 μm in an amount of 1 to 30% by quantity, and particles having a particle diameter not less than 16 μm in an amount of not greater than 2% by volume. Further, the non-magnetic toner has a volume average particle diameter of from 4 to 10 μm. The toner also satisfies the relationship: N/V=0.04N+k, wherein N represents the percentage by quantity of the toner particles having a particle diameter not greater than 5 μm and is a number in the range of from 17 to 60, V represents the percentage by volume of the toner particles having a particle diameter not greater than 5 μm, and k represents a number in the range of from 4.5 to 6.5.
As prior art focusing on the toner pulverization method, unexamined published Japanese Patent Application No. (hereinafter referred to as JOP) 8-10350 discloses a toner pulverizing method which can manufacture a toner without causing toner adhesion and toner fusion bonding in a pulverizer, and which is efficient in terms of power consumption. The toner pulverizing method includes the steps of preliminary pulverizing a toner composition mixture with a mechanical pulverizer to obtain a toner powder having a particle diameter of from 20 to 60 μm, and then pulverizing the toner powder with a counter air pulverizer to prepare the toner.
In attempting to improve the product yield of a toner in the pulverization and the granulation steps, JOP 5-313414 discloses a toner manufacturing method including the steps of preliminarily pulverizing a toner composition mixture with a mechanical pulverizer to obtain a toner powder having an average particle diameter of from 20 to 100 μm.
While the methods described in JOPs 8-10350 and 5-343414 are effective in solving some problems in toner manufacturing mentioned above, the quality of images produced by the toners obtained by theses methods is inferior to that in the case where a toner manufactured by polymerization is used.
The inventors of the present invention have studied conventional toner manufacturing methods and have found that, when a small-sized toner having a toner particle of from 4 to 6 μm is manufactured by a conventional method, the main problem which occurs is that super fine toner particles having a particle diameter not greater than 2 μm is present in a large amount.
Such super fine toner particles having a particle diameter not greater than 2 μm causes the following problems even when the content thereof is low.    (1) Even when the content of super fine toner particles having a particle diameter not greater then 2 μm is low, the super fine toner particles cover the surface of a carrier particle so completely as to lower the chargeability of the carrier, and thereby the furnished toner particles cannot be charged sufficiently. As a result, a problem occurs in that the toner scatters when images are sequentially output.    (2) The super fine toner particles having a particle diameter not greater than 2 μm are extremely small and thus tend to strongly adhere to carrier particles, resulting in formation of spent toner on the carrier particles, thereby deteriorating the charging ability of the developer containing the toner.    (3) The super fine toner particles having a particle diameter not greater than 2 μm tend to form a film on an image bearing member, a developing sleeve, etc.    (4) It is well known that, when large coarse toner particles are present, isolated dot images are not exactly reproduced in the developing process and the transfer process and resultingly the images obtained look rough, i.e., images having non-uniform density. The inventors of the present invention have found that faithful reproduction of such isolated dots in the developing process is obstructed by the super fine toner particles having a particle diameter not greater than 2 μm. Although the cause of this phenomenon is unclear, it is believed that the super fine toner particles having a particle diameter not greater than 2 μm are extremely different in terms of adhesion strength and charging ability, etc. Therefore, the super fine toner particles may affect behaviors of other toner particles and cause non-uniform reproduction of isolated dots at the time of development especially when the super fine toner particles adhere to an image bearing member.
In a toner manufacturing method using the conventional pulverization mentioned above, these problems are not satisfactorily solved.
When a toner having an average particle diameter of from 4 to 6 μm is manufactured by a toner manufacturing method using the conventional pulverization mentioned above, its particle diameter is small so that the quality of images obtained is improved to a certain degree. However, the quality of images obtained is not comparable to that of the images obtained by a toner manufactured by a polymerization method. This is considered to be because super fine toner particles having a particle diameter not greater than 2 μm are present.
Now the reason why super fine toner particles having a particle diameter not greater than 2 μm are difficult to remove by classification is described with reference to FIG. 1.
A particle m receives a centrifugal force F by an air stream created by rotation of a classification rotor. The velocity V of air stream created by the rotation is determined by the radius r of the classification rotor and the number of rotation R of the rotor. In the case of a toner particle having a small toner particle diameter, the particle m is small and therefore hardly receives the centrifugal force F. Thus a toner having a small particle diameter is not well classified.
Especially, a super fine toner particle having a toner particle diameter not greater than 2 μm is hardly influenced by the centrifugal force F, and therefore is not sufficiently removed even in a fine particle classification process following the above classification.
According to the studies by the inventors of the present invention, among the conventional toner manufacturing methods mentioned above, especially the pulverization method using a counter air pulverizer produces a great amount of super fine powder having a particle diameter not greater than 2 μm at the time of pulverization. Therefore it is extremely difficult to eliminate the super fine toner particles in the fine powder classification process performed after the classification process.
This is not only simply because the toner particle is small sized, but also because it is believed that charge sites present on the surface of a pulverized toner particle are removed by a counter air pulverizer. Thus the resultant super fine toner particles tend to have an extremely large amount of charge and strong adhesion properties relative to super fine toner particles produced by other pulverization methods.
The problem discussed above caused by super fine powder having a particle diameter not greater than 2 μm is a serious problem especially for high speed continuous image outputs and color image outputs.
Specifically, when a high speed continuous printing is performed, isolated dot toner images on an image bearing member tend to be destroyed, such that the granularity of the toner images deteriorates, resulting in deterioration of image quality.
In addition, replenished toner particles have insufficient charging properties, which leads to decrease in image density, deterioration in fine line reproducibility and occurrence of background fouling.
Further, since four color toner layers are overlaid in forming a color image, the problem mentioned above becomes serious, and image quality seriously deteriorates.
The toner manufactured by counter air pulverization has a rounded form. Therefore, theoretically, the toner is expected to have good transferability which is comparable to that of a toner manufactured by polymerization. However, when toner particles having an average particle diameter of from 4 to 6 μm are manufactured, super fine toner powder having a particle diameter not greater than 2 μm inhibits improvement of image quality. Therefore, it is believed that elimination of such super fine toner powder having a particle diameter not greater than 2 μm will lead to improvement on the quality of images produced using a toner manufactured by pulverization.
Because of these reasons, a toner is described which is manufactured by pulverization, but from which super fine powder having a particle diameter not greater than 2 μm is eliminated, to an extent such that toner image quality is improved and the toner can be used for the progressed hardware for use in electrophotography.